WASHINGTON — With one suspect dead and the other captured and lying grievously wounded in a hospital, the investigation into the Boston Marathon bombings turned on Saturday to questions about the men’s motives, and to the significance of an overseas trip one of them took last year.

Federal investigators are hurrying to review a visit that one of the suspected bombers made to Chechnya and Dagestan, predominantly Muslim republics in the north Caucasus region of Russia. Both have active militant separatist movements. Members of Congress expressed concern about the F.B.I.’s handling of a request from Russia before the trip to examine the man’s possible links to extremist groups in the region.

Tamerlan Tsarnaev, who died early Friday after a shootout with the police in Watertown, Mass., spent six months in Dagestan in 2012, and analysts said that sojourn might have marked a crucial step in his alleged path toward the bombings.

Kevin R. Brock, a former senior F.B.I. and counterterrorism official, said, “It’s a key thread for investigators and the intelligence community to pull on.”

The investigators began scrutinizing the events in the months and years before the fatal attack, as Boston began to feel like itself for the first time in nearly a week.

On Monday, the twin bombings near the finish line of the Boston Marathon killed three people and wounded more than 170. The tense days that followed culminated in Friday’s lockdown of the entire region as the police searched for Mr. Tsarnaev’s younger brother from suburban backyards to an Amtrak train bound for New York City.

Dzhokhar Tsarnaev, 19, was taken into custody Friday night after he was found, bloody and weakened, hiding on a boat in a driveway in Watertown. He was still too wounded to speak on Saturday, said Gov. Deval Patrick of Massachusetts. Special counterterrorism agents trained in interrogating high-value detainees were waiting to question him, according to a law enforcement official. An issue arose about the administration’s decision to question him for a period without giving him a Miranda warning, under an exception for questions about immediate threats to public safety.

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A news conference in Boston on Thursday during which images of the two bombing suspects were released to the public.CreditGretchen Ertl for The New York Times

The brothers’ motives are still unclear. Of Chechen heritage, they had lived in the United States for years, according to friends and relatives, and no direct ties have been publicly established with known Chechen terrorist or separatist groups. While Dzhokhar became a naturalized American citizen last year, Tamerlan was still seeking citizenship. Their father, Anzor, said Tamerlan had made last year’s trip to renew his Russian passport.

The significance of the trip was magnified late Friday when the F.B.I. disclosed in a statement that in 2011 “a foreign government” — now acknowledged by officials to be Russia — asked for information about Tamerlan. The request was “based on information that he was a follower of radical Islam and a strong believer, and that he had changed drastically since 2010 as he prepared to leave the United States for travel to the country’s region to join unspecified underground groups.”

The senior law enforcement official said the Russians feared he could be a risk, and “they had something on him and were concerned about him, and him traveling to their region.” Chechen extremists pose a greater threat to Russia than they do to the United States, counterterrorism specialists say, though some of the groups have had ties to Al Qaeda.

But the F.B.I. never followed up on Tamerlan once he returned, a senior law enforcement official acknowledged on Saturday, adding that its investigation did not turn up anything and it did not have the legal authority to keep tabs on him. Investigators are now scrambling to review that trip, and learn about any extremists who might have influenced, trained or directed Tamerlan while he was there.

President Obama and Republican lawmakers devoted their weekly broadcast addresses to the Boston attack, with both sides finding a common voice. Mr. Obama also met with his national security team for an update on the investigation.

“Americans refuse to be terrorized,” Mr. Obama said. “Ultimately, that’s what we’ll remember from this week.”

Since 1994, Russia and the United States have routinely exchanged requests for background information on residents traveling between the two countries on visa, criminal or terrorism issues.

The F.B.I. responded to the request in 2011 by checking “U.S. government databases and other information to look for such things as derogatory telephone communications, possible use of online sites associated with the promotion of radical activity, associations with other persons of interest, travel history and plans, and education history,” it said in a statement.

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Investigators worked near the location where a suspect in the marathon bombings was arrested.CreditMatt Rourke/Associated Press

In January 2011, two counterterrorism agents from the bureau’s Boston field office interviewed Tamerlan and family members, a senior law enforcement official said on Saturday. According to the F.B.I.’s statement, “The F.B.I. did not find any terrorism activity, domestic or foreign,” and conveyed those findings to “the foreign government” by the summer of 2011.

Federal officials said on Saturday that the Department of Homeland Security, however, had decided not to grant a petition from Tamerlan for United States citizenship after officials found a record in his files that he had been interviewed by the F.B.I. His petition was held for further review.

As the law enforcement official put it, “We didn’t find anything on him that was derogatory.”

The Russian state news agency RIA Novosti quoted the father of the Tsarnaev brothers recalling the F.B.I.’s close questioning of his elder son, “two or three times.” He said they had told his son that the questioning “is prophylactic, so that no one sets off bombs on the streets of Boston.”

In an interview in Russia, Zubeidat Tsarnaeva, the mother of the two men, said that the agents had told her that Tamerlan was “an excellent boy,” but “at the same time, they told me he is getting information from really extremist sites, and they are afraid of him.”

After Tamerlan’s visit to Dagestan and Chechnya, signs of alienation emerged. One month after he returned to the United States, a YouTube page that appeared to belong to him was created and featured multiple jihadist videos that he had endorsed in the past six months. One video featured the preaching of Abdul al-Hamid al-Juhani, an important ideologue in Chechnya; another focused on Feiz Mohammad, an extremist Salafi Lebanese preacher based in Australia. He also created a playlist of songs by a Russian musical artist, Timur Mucuraev, one of which promoted jihad, according to the SITE Intelligence Group, which monitors statements by jihadists.

The Boston bombings have led to increased cooperation between Washington and Moscow, a jarring shift coming amid weeks of rancor over American criticism of Russia’s human rights record. Presidents Obama and Vladimir V. Putin spoke by telephone late Friday night, in a conversation initiated by the Russian side, the Kremlin announced. The Kremlin’s statement said both leaders expressed “the building of close coordination between Russian and American intelligence services in the battle with global terrorism.”

Nevertheless, there were glaring questions about the case, among them how Tamerlan had escaped scrutiny.

A Russian intelligence official told the Interfax news service on Saturday that Russia had not been able to provide the United States with “operatively significant” information about the Tsarnaev brothers, “because the Tsarnaev brothers had not been living in Russia.”

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On the streets of Watertown, Mass., residents and police officers celebrated after a suspect was brought into custody.

Andrei Soldatov, an investigative journalist who specializes in Russia’s security services, said he believed that Tamerlan might have attracted the attention of Russian intelligence because of the video clips he had posted under his own name, some of which were included on a list of banned materials by the Federal Security Service, or F.S.B.

On Saturday morning, federal prosecutors were drafting a criminal complaint against Dzhokhar Tsarnaev, who was wounded in the leg and neck.

An official said the criminal complaint would most likely include a constellation of charges stemming from both the bombings and the shooting, possibly including the use of weapons of mass destruction, an applicable charge for the detonation of a bomb. That charge, the official said, carries a maximum penalty of death. Though Massachusetts has outlawed the death penalty, federal law allows it.

The F.B.I. and local law enforcement agencies continued on Saturday to gather evidence recovered from the suspects’ home and the cars they used. Investigators found five pipe bombs and three grenades after the firefight Friday, and they were seeking to identify the origins of the explosives.

Agents fanned out to interview family members and others who knew the brothers to determine any motive, as well as clues about what or who radicalized them. Three Kazakh citizens who were acquainted with Dzhokhar Tsarnaev contacted the Kazakh Embassy in Washington, reporting that they had been questioned by the F.B.I. and asking for consular assistance, said Ilyas T. Omarov, a spokesman for the Foreign Ministry of Kazakhstan, a former Soviet republic. None of the three were held, he said.

Muslim leaders in many cities rushed to hold news conferences and preach sermons at mosques denouncing the bombing suspects, mourning the victims and praising the response of law enforcement and the community in Boston. They were eager to dissociate their faith from the Muslim suspects, and to head off a backlash against Muslims in the United States.

Anzor Tsarnaev and his younger son first came to the United States legally in April 2002 on 90-day tourist visas, federal law enforcement officials said. Once in this country, the father applied for political asylum, claiming he feared deadly persecution based on his ties to Chechnya. Dzhokhar, who was 8, applied for asylum under his father’s petition, the officials said.

Tamerlan Tsarnaev came to the United States later, and applied for American citizenship on Sept. 5 last year, federal law enforcement officials said.

Correction:April 20, 2013

An earlier version of this article misidentified the office held by Tim Scott of South Carolina. He is a senator, not a representative.

Eric Schmitt and Michael S. Schmidt reported from Washington, and Ellen Barry from Moscow. Reporting was contributed by John Schwartz and Julia Preston from New York; Andrew Roth and David M. Herszenhorn from Makhachkala, Dagestan; Peter Baker from Washington; and C. J. Chivers from the United States.

A version of this article appears in print on , on Page A1 of the New York edition with the headline: Bombing Inquiry Turns to Motive and Russia Trip. Order Reprints | Today’s Paper | Subscribe